You said the plane is out of annual, and he took you up for a flight. Not a prudent thing.
His asking price of $37,000 is too much. Granted that he paid $15,000 and spent another $22,000 fixing it up, but he has got 3-4 years of use from it, so its value must be less. Like I mentioned in my email to you, each hour of engine usage is worth about $6, and each hour of airframe is worth about 50 cents.
I'd be interested to know what he spent that $22,000 on. A remanufactured O-200 only costs around $12K-$13K. Where did the other $9K-$10K go? If he counts the costs of annuals/maintenance during the last 3-4 yrs in there, subtract them out; those expenses are UPKEEP, they don't count as IMPROVEMENTS which increase the value of the plane.